Tumors are diverse and heterogeneous, but all share the ability to proliferate without control. Deregulated cell proliferation coupled with suppressed apoptotic sensitivity constitutes a minimal requirement upon which tumor evolution occurs.
Apoptosis is the process by which cells enter programmed cell death, a vital phenomenon that takes place during development, and is essential for the maintenance of homeostasis. The biochemical event that is believed to irreversibly commit a cell to apoptosis is the activation of caspases (cysteine proteases cleaving after aspartic residues). Cells undergoing apoptosis display characteristic morphological and biochemical changes, including membrane blebbing, cell rounding, chromatin condensation, DNA cleavage, expression of apoptotic markers at the cell surface and inhibition of anti-apoptotic signaling pathways. All these events can be blocked by specific caspase inhibitors. It is thus the cleavage of the caspase substrate that is responsible for most, if not all, of the characteristic changes observed during apoptosis.
The execution phase of apoptosis is triggered when caspase substrates in a cell are cleaved. Dozens of caspase substrates have been identified and the list is growing steadily (Earnshaw W. C. et al., “Mammalian caspases: structure, activation, substrates, and functions during apoptosis” Annu. Rev. Biochem. 68, 383, 1999). Once cleaved, caspase substrates mediate the biochemical and morphological events observed during apoptosis such as amplification of the activation of caspases, DNA fragmentation, nuclear breakdown, etc.
Furthermore, Mitogen-activated protein kinase (MAPK) pathways have been shown to regulate apoptosis in a positive or negative manner (Jarpe M. B. et al., “Anti-apoptotic versus pro-apoptotic signal transduction: checkpoints and stop signs along the road to death” Oncogene, 17, 1475, 1998; Widmann C. et al, “Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human” Physiol. Rev. 79, 143, 1999). This could explain why the apoptotic caspases target some of the signaling proteins that regulate MAPK and/or are components of MAPK pathways (Widmann C. et al., “Caspase-dependent cleavage of signaling proteins during apoptosis. A turn-off mechanism for anti-apoptotic signals” J. Biol. Chem., 273, 7141, 1998). These proteins include MEKK1, PAK2, Mst1 and RasGAP.
Recently, Yang and Widmann, (Yang J.-Y. and Widmann C., “Antiapoptotic signaling generated by caspase-induced cleavage of RasGAP” Mol. Cell. Biol., 21, 5346, 2001; “A subset of caspase substrates functions as the Jekyll and Hyde of apoptosis” Eur. Cytokine Netw., 13, 387, 2002a; “The RasGAP N-terminal fragment generated by caspase cleavage protects cells in a Ras/PI3K/Akt-dependent manner that does not rely on NFkappa B” J. Biol. Chem., 277, 14641, 2002b), have demonstrated that RasGAP, a regulator of Ras and Rho GTP-binding proteins, is an unconventional caspase substrate because it can induce both anti- and pro-apoptotic signals, depending on the extent of its cleavage by caspases. They have shown that at low levels of caspase activity, RasGAP is cleaved at position 455, generating an N-terminal sequence (sequence N) and a C-terminal sequence (sequence C).
Sequence C, but not full-length RasGAP, induced a strong apoptotic response in HeLa cells as assessed by its ability to induce the appearance of pycnotic nuclei, activation of caspase 3, and cleavage of PARP.
In the same study, the authors have also shown that sequence N, rather than promoting cell death, appears to be a general blocker of apoptosis downstream of caspase activation. At higher levels of caspase activity, the ability of sequence N to counteract apoptosis is suppressed when it is cleaved at position 157. This latter cleavage event generates two sequences, N1 and N2, that in contrast to sequence N, have been shown to seitizises cells which can develop high caspase activities toward apoptosis induced by cisplatin, a drug used in chemotherapy to treat cancers.
However, it has been shown in Leblanc et al (Leblanc V. et al., “Ras-GTPase activating protein inhibition specifically induces apoptosis of tumour cells” Oncogene, 18, 4884, 1999) that injection of a monoclonal antibody directed against the SH3 domain of the N2 sequence of RasGAP in order to inhibit this protein specifically induces apoptosis in cancer cells. It is also know from patent application WO99/65947 (Parker et al.) that monoclonal antibodies directed against a RasGAP SH3-domain-binding protein, G3BP, induce apoptosis in cancer cells in which G3BP is specifically overexpressed.
These results seem to indicate that the RasGAP pathway regulating growth, through the RasGAP SH3 domain, is essential for some cancer cells to survive. These findings seem to be in contrast with the results obtained by Yang and Widmann thus leading to the conclusion that the RasGAP SH3 domain has a quite ambivalent function in the induction and regulation of apoptosis in cells.
Chemotherapy, alone or in combination with other treatments (e.g. radiotherapy), are currently one of the most common and efficient therapeutical tool to treat cancers. The efficacy of the drugs used in chemotherapy to treat cancers relies on their ability to kill cancer cells. There is, however, a limitation in the use of these drugs that comes from the fact that they can also adversely affect normal cells, non cancer cells, since they not only induce a strong stimulation of caspases in cancer cells but also in normal cells, non cancer cells, especially those cells that divide quickly.
Therefore, the challenge for the clinicians is to choose the doses of drugs that are high enough to eliminate the tumors but not so high as to induce severe side effects in the patients such as hair loss, nausea and vomiting, cardiac toxicity and secondary cancers.
Improving the selectivity of drugs towards cancer cells would obviously increase the efficacy of chemotherapeutic treatments thereby enabling lower doses of drugs. This will also result in reducing as far as possible the severe above-listed side effects.
Therefore, the object of the present invention is to provide an improved approach, in combination with a drug, for the treatment or prevention of cancers, which does not have the above-mentioned drawbacks.